A turbomolecular pump uses the operation of turbine blades that combine rotors and stators to create a vacuum by evacuation. Turbine blades are radially formed about a rotational shaft so that the circumferential velocity is different between the base portion of the blade and the tip portion of the blade. Because of this, the design is optimized so that the performance as defined by the blade angle and the distance between the blades at an intermediate point between the blade base and the blade tip achieves the target performance.
However, if turbine blades are constructed of flat plates as previously done, at points located more distally than an intermediate point, the increase in the aperture rate becomes greater than the increase in the circumferential velocity. This increases the effects of reverse flow as compared to the effects at an intermediate point, undermining the optimum design. With the present specification, the rate by which the opposite side is visible when looking down the axial direction of the turbine blade is referred to as the aperture rate.
Because of this, twisted blades have been proposed where the blade angle of the turbine blade gradually decreases from the blade base towards the blade tip so as to prevent the increase in the aperture rate at the outer blades (see for example Patent Literature 1).    Patent Literature 1: Unexamined Patent Application Publication 02-61387